Patent Grant
6927368
1. Field of the Invention
The present invention relates to a method and apparatus for controlling power to an electrical load. More particularly, the present invention relates to a method and apparatus for controlling power to a high power heater element in a fuser of an imaging device to provide improved warm-up and temperature control characteristics.
2. Description of Related Prior Art
In printing, the amount of time it takes for the first page of a print job to be printed and to reach the printer's output bin is known as first copy time, and is an important feature to users of the printer. In conventional electrophotographic printers, the controlling factor for first copy time typically has been the amount of time it takes to warm up a cold fuser to a target temperature for performing a fusing operation.
To optimize the first copy time, the fuser must be heated as fast as possible. In addition, it is necessary to maintain the temperature of the fuser within a narrow temperature window close to a predetermined target temperature for a given mode of operation of the fuser. These requirements impose conflicting design constraints on a heater element incorporated within a heated fuser roll. For example, it is desirable to have a relatively high power heater element to provide a fast temperature ramp up when initially heating the fuser. On the other hand, when controlling power to such a high power heater element, it is difficult to operate within a narrow temperature window, particularly when small, controlled temperature corrections are required to maintain a target temperature.
A further limitation on the operation of the heater element relates to noise reduction requirements imposed in Europe on all electrical and electronic equipment, known as the “harmonic” requirement IEC 61000-3-2, and the “flicker” requirement IEC 61000-3-3. When power is first applied to the heater element for the fuser, such as a 750 W tungsten-filament lamp or other high wattage lamp, there is typically a large inrush current that primarily produces harmonic noise and an instantaneous voltage drop that can affect other electrical equipment connected to the same or a nearby electrical branch circuit. The effect of the sudden inrush current at the heater element, and associated voltage drop, is readily noticeable as a flicker in the output of fluorescent lights. As the temperature of the heater element rises, its resistance also increases and a larger amount of energy may be applied without the substantial voltage variations experienced during initial warm-up.
One proposed solution to the flicker problem is to control a fuser by using on-off control, i.e., switching power to the fuser heater element on and off, to provide a desired temperature change in the fuser. For example, U.S. Pat. No. 6,097,006 discloses apparatus for increasing the temperature of a fuser in which a switching unit is turned on and off to intermittently disrupt the current supplied to the fuser to warm up the fuser wherein the duration of the “on” relative to the “off” time is selected to provide a desired temperature increase and to control the generation of flicker.
In an alternative approach, U.S. Pat. No. 6,111,230, assigned to the assignee of the present application, discloses a method and apparatus for energizing an electrically driven apparatus that applies power to the apparatus by using phase-angle control. Triggering of the AC power is delayed for each half cycle of the AC current waveform, and in particular is initially delayed by nearly the entire half cycle. The delay time is then decreased at a predetermined rate before triggering each subsequent half cycle until full power is reached.
There is a continuing need to provide a reduced warm-up time for fusers, and in particular to provide a reduced warm-up time for fusers in color laser printers, where the fuser rolls are commonly formed of an aluminum core coated with silicon rubber, having a lower thermal conductivity than the aluminum core, and covered with a fluoropolymer sleeve. The desired reduction in warm-up time may be achieved by providing a high power heater, for example, higher than approximately 800 watts for a single lamp system and 750 watts for a two-lamp system. However, the use of these high power heater elements is dependent on meeting the above-mentioned European harmonic and flicker requirements on electrical equipment. Further, use of such high power heater elements is additionally contingent on providing a control method capable of maintaining the fuser temperature within a narrow range of predetermined target temperatures, such as are defined by target standby and print mode temperatures.
A method of controlling power applied to an electrical load is provided by the present invention whereby the application of power meets European harmonic and flicker requirements. In particular, the present invention provides a power control method which is adapted to be used for supplying power to a fuser having a high power heater element, and operates to provide improved warm-up characteristics, as well as improved temperature control maintaining an operating temperature of the fuser within a narrow temperature window.
In accordance with one aspect of the invention, a method of controlling power to an electrical load is provided comprising supplying power from a power source; modulating an output from the power source to provide power at a first modulated power level to power the electrical load; and modulating the first modulated power level to control the power provided to the electrical load at the first modulated power level in accordance with a second modulated power level.
In accordance with another aspect of the invention, a method of controlling power to a heater element is provided comprising supplying AC current from a power source; producing a waveform pulse width modulation control signal to define a first modulated power level to power the heater element; and producing a duty cycle pulse width modulation control signal to define a second modulated power level to control application of the first modulated power level to the heater element.
In accordance with yet another aspect of the invention, a method of controlling power to a heater element is provided comprising supplying power from a power source; sensing a temperature controlled by the heater element; comparing the sensed temperature to a predetermined temperature; supplying power to the heater element in accordance with a first switching signal providing a first set of power level control parameters when the sensed temperature is below the predetermined temperature; and supplying power to the heater element in accordance with a second switching signal providing a second set of power level control parameters when the sensed temperature is above the predetermined temperature wherein the power supplied to the heater element in accordance with the second set of power level control parameters is reduced from the power supplied by the first set of power level control parameters.
In accordance with still another aspect of the invention, a method of controlling power to an electrical load is provided comprising supplying power from a power source; controlling supply of the power to an electrical load in accordance with a duty cycle pulse width modulation signal for providing a periodic application of power at a predetermined power level; providing a preheat defined by a lower power level than the predetermined power level; and wherein the preheat is provided prior to individual periods of the periodic application of power.
In accordance with a further aspect of the invention, a method of controlling power to a heater element in an electrical device is provided comprising supplying power from a power source to said heater element; defining a high threshold temperature for said electrical device; determining a temperature of said electrical device above said high threshold temperature to define a low power region; and continuing to supply power to said heater element in said low power region while causing a decrease in the temperature of said electrical device.
In accordance with still a further aspect of the invention, a method of controlling a heater element in an electrical device is provided comprising defining a target temperature for said electrical device; and supplying power from a power source to heat said electrical device from substantially a room temperature wherein said power is applied at a first power level during a first stage up to a first predetermined temperature less than said target temperature, said power is supplied at a second power level, less than said first power level, during a second stage up to a second predetermined temperature greater than said first predetermined temperature and less than said target temperature, and said power is applied at a third power level, less than said second power level up to said target temperature.
In accordance with yet a further aspect of the invention, heating control apparatus for connecting and disconnecting AC power from an AC power source at zero crossings of the AC power is provided comprising a switching device that is selectively turned on and off; a heater element connected to the AC power source via the switching device; a zero-cross driving circuit for driving the switching device at zero-cross points of the power source; and control means providing a dual pulse width modulation control signal for controlling the driving circuit, the signal being asynchronous with the AC power whereby the switching device is turned on and off for half cycles of the AC power corresponding to the signal, the dual pulse width modulation control signal comprising a first waveform component providing selected half cycles of the AC power, and a second duty cycle/period signal component providing the selected half cycles of the AC power for a selected duty cycle portion of a time period.
Other features and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.
The present application provides a method of controlling a high power heater element in an electrical device. As a non-limiting example presented for illustration of the operating principals of the present application, the electrical device may comprise a fuser 1 such as is provided in an electrophotographic imaging device (EID), a portion of which is shown in
As will be described in further detail below, AC power is supplied to the heater element 2 using one of a plurality of waveforms, first modulated power levels, as selected by a first pulse width modulation (PWM) control signal or waveform PWM control signal. Each waveform is defined by a waveform length and a waveform power segment, the waveform length comprising a predetermined number of half cycles and the waveform power segment comprising a selected number of the half cycles of the waveform length during which power is supplied to the heater element. Each of the plurality of waveforms provide a discrete power level that is periodically repeated based on a period equal to the waveform length. In a prototype embodiment of the power control of the present application, a waveform length of up to fifteen could be selected such that fifteen waveform power segments, ranging from one-out-of-fifteen half cycles to fifteen-out-of-fifteen half cycles, could be selected. It is contemplated that any reasonable waveform length could be used, i.e., a waveform length equal to a number of half cycles greater than or less than fifteen.
In the illustrated embodiment of the present application, waveform length was selected as three so that the plurality of waveforms provide three discrete power levels that are periodically repeated on a period of three half cycle segments of the cyclical AC power waveform and comprise: 1) a one-out-of-three half cycle waveform 10 (FIG. 2), as controlled by a 1/3 waveform PMW control signal 12 where power is supplied to the heater element one-out-of-three half cycles to provide one third power; 2) a two-out-of-three half cycle waveform 14 (FIG. 3), as controlled by a 2/3 waveform PMW control signal 16, where power is supplied two-out-of-three half cycles to provide two thirds power, and; 3) a three-out-of-three half cycle waveform 18 (FIG. 4), as controlled by a full or 3/3 waveform PWM control signal 20, where power is supplied three-out-of-three half cycles to provide full power.
The half cycle power on times are depicted as solid lines in
Referring to Table 1, the effect of filament temperature on inrush current for a 1000 W heater filament is illustrated, where dmax describes one of the European requirements for controlling flicker, dmax being a percentage measure of the change of voltage per unit of time.
The limit for dmax, as set by the European standard, is 4% and it can be seen that when the heater element is at 22° C. (72° F.), or approximately room temperature, the inrush current causes dmax to exceed this limit. Further, if the heater element 2 is turned on with full power when the temperature of the heater element in this example is at any temperature lower than a lower limit temperature of approximately 220° C. (432° F.), the dmax limit will be exceeded. Observing that the amount of inrush current reduces with increasing temperature of the heater element, the present application supplies power to the heater element at a level corresponding to a first PWM waveform control signal which is controlled through selection of a particular duty cycle and period corresponding to a duty cycle PWM control signal. This power control avoids exceeding the flicker requirement while rapidly heating the heater element 2. Specific examples of power control of the present application will now be described.
Whenever power is initially applied to a heater filament, an inrush current is generated that may result in light flicker, and is particularly evident when applying power to the cold filament of a high power heater. The temperature of a heater filament is subject to large variations sin short time periods, such that when power to the filament is shut off, the filament's temperature may drop below the lower limit temperature for dmax within a few seconds. On the other hand, when power is applied to the heater filament, its temperature will quickly rise to a level above the lower limit temperature for dmax, typically in less than a second. Thus, it is possible for the current in the heater filament to exceed the dmax limit, with resulting light flicker, any time during operation of the fuser when the heater element is turned off and subsequently turned back on, including during periodic cycling of the heater element as it is turned off and on, such as during a print job or other powered operations of the fuser.
Referring to
Referring to
For the purposes of the explanation provided below, power regions are defined for application of particular power levels and power outputs of the heater element wherein a high power region, TH, is triggered when the temperature of the heated fuser roll goes below a low threshold, e.g., a target temperature minus 1° C.; and a low power region, TL, is triggered when the temperature of the heated fuser roll goes above a high threshold, e.g., a target temperature plus 1° C. Power application in the high power region, TH, and the low power region, TL, is controlled by a power switching signal 24 (see FIGS. 10 and 11). The power switching signal 24 is set to 1 during the high power region corresponding to a first set of power level control parameters listed in row TH in the table of
The application of power to the heater element during the low and high power regions will vary depending upon the mode of operation of the fuser, such that separate subsets of power level control parameters are provided for the print mode of operation and the standby mode of operation. Additionally, the power level control parameter subsets are comprised of two control components including a waveform component, determined by the waveform PWM control signal, and a duty cycle/period component, determined by the duty cycle PWM control signal. The subsets of parameters in row TL are selected to provide for a decrease in temperature with application of power to the fuser for the particular mode of operation, and the subsets of parameters in row TH are selected to provide for an increase in temperature with application of power to the fuser for the particular mode of operation.
As noted above, power is provided to the heater element during the print mode applying either the one-out-of-three waveform 10 or the two-out-of-three waveform 14. The particular waveform applied depends on whether the temperature of the heated fuser roll is below the target print temperature, such that the high power region power level control parameters are applied, or the temperature of the heated fuser roll is above the target print temperature, such that the low power region power level control parameters are applied.
Consider first the low power region, TL, corresponding to the fuser roll temperature exceeding the target temperature. It is necessary for the fuser roll to cool toward the target temperature during this time, and the waveform PWM control signal applies the one-out-of-three waveform 10 to supply a reduced level of power to the heater element. It should be noted that even during cooling of the fuser roll, the duty cycle PWM control signal operates to periodically apply power to the heater element (during the duty cycle portion of the period), and thereby maintain the filament in a warm state while permitting the temperature of the heated fuser roll to decrease. Further, it should be understood that, based on the relationship between inrush current and the heater filament temperature shown in Table 1 above, there exists a maximum cooling time for a given heater element to meet the dmax requirement. If the cooling or off time of the heater element exceeds this maximum cooling time, the inrush current produced as a result of the low heater element resistance may cause the dmax limit, as specified by the European standards, to be exceeded, thus producing a noticeable affect. Accordingly, if the period is too long and the duty cycle is sufficiently small, a noticeable flicker may occur with the repeated or periodic application of power during the duty cycle. Therefore, to ensure that the dmax value remains below the flicker standard, the period of the duty cycle PWM control signal is set to be shorter than the maximum cooling time.
A specific non-limiting example of application of power to the heater element during operation in the print mode is illustrated in
The high power region, TH, is similar to the low power region, TL, in that the period of the duty cycle PWM control signal is set shorter than the maximum cooling time for the heater element. The switching signal 24 is set to 1 to cause the two-out-of-three waveform 14 to be applied to supply power to the heater element. In the illustrated example of
It should be apparent that accurate selection of the duty cycle is important in that selection of a duty cycle that is too long may result in temperature overshoot, as too much power may be applied to the fuser roll in a short time. Alternatively, selection of a duty cycle that is too short may not meet the power requirements for maintaining the fuser roll near its target temperature during a print fusing operation. Accordingly, the duty cycle for the high power region, TH, must be carefully selected to provide a narrow temperature operating window under all operating conditions wherein the operating window for the illustrated example is approximately 4° C., i.e., 2° C. above and 2° C. below the target temperature, for operation in both the high and the low power regions.
Power is supplied to the heater element by applying the one-out-of-three waveform 10 to the heating element for both the low power region, TL, and the high power region, TH, during the standby mode. However, different periods and duty cycles are defined by the duty cycle PWM control signal for the different power regions. Specifically, as seen in
Referring to
From the above description, it should be apparent that the present application provides a dual pulse width modulation control method whereby power to a high power electrical component may be accurately controlled while minimizing adverse affects of flicker and harmonics associated with prior power control arrangements. Further, the combined use of two PWM controls in the present application permits a fuser design incorporating a high power heater element for providing reduced fuser warm-up times while also enabling improved temperature control for operation within a narrow temperature window for improved print quality.
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